isolationisolation isolation via virtual machines dan boneh virtual machines virtual machine monitor...
Post on 12-Jun-2020
28 Views
Preview:
TRANSCRIPT
DanBoneh
Isolation
Theconfinementprinciple
CS155:ComputerSecurity
DanBoneh
Running untrusted codeWe often need to run buggy/unstrusted code:
– programs from untrusted Internet sites:• apps, extensions, plug-ins, codecs for media player
– exposed applications: pdf viewers, outlook
– legacy daemons: sendmail, bind
– honeypots
Goal: if application “misbehaves” ⇒ kill it
DanBoneh
Approach: confinementConfinement: ensure misbehaving app cannot harm rest of system
Can be implemented at many levels:– Hardware: run application on isolated hw (air gap)
⇒ difficult to manage
airgap network1Network2
app1 app2
DanBoneh
Approach: confinementConfinement: ensure misbehaving app cannot harm rest of system
Can be implemented at many levels:– Virtual machines: isolate OS’s on a single machine
VirtualMachineMonitor(VMM)
OS1 OS2
app1 app2
DanBoneh
Approach: confinementConfinement: ensure misbehaving app cannot harm rest of system
Can be implemented at many levels:– Process: System Call Interposition
Isolate a process in a single operating system
OperatingSystem
process2
process1
DanBoneh
Approach: confinementConfinement: ensure misbehaving app cannot harm rest of system
Can be implemented at many levels:– Threads: Software Fault Isolation (SFI)
• Isolating threads sharing same address space
– Application: e.g. browser-based confinement
DanBoneh
Implementing confinementKey component: reference monitor– Mediates requests from applications
• Implements protection policy• Enforces isolation and confinement
– Must always be invoked:• Every application request must be mediated
– Tamperproof:• Reference monitor cannot be killed• … or if killed, then monitored process is killed too
– Small enough to be analyzed and validated
DanBoneh
A old example: chrootOften used for “guest” accounts on ftp sites
To use do: (must be root)
chroot /tmp/guest root dir “/” is now “/tmp/guest”su guest EUID set to “guest”
Now “/tmp/guest” is added to file system accesses for applications in jailopen(“/etc/passwd”, “r”) ⇒
open(“/tmp/guest/etc/passwd” , “r”)
⇒ application cannot access files outside of jail
DanBoneh
JailkitProblem: all utility progs (ls, ps, vi) must live inside jail
• jailkit project: auto builds files, libs, and dirs needed in jail env• jk_init: creates jail environment• jk_check: checks jail env for security problems
• checks for any modified programs,• checks for world writable directories, etc.
• jk_lsh: restricted shell to be used inside jail
• note: simple chroot jail does not limit network access
DanBoneh
Escaping from jailsEarly escapes: relative paths
open( “../../etc/passwd”, “r”) ⇒
open(“/tmp/guest/../../etc/passwd”, “r”)
chroot should only be executable by root.– otherwise jailed app can do:• create dummy file “/aaa/etc/passwd”• run chroot “/aaa”• run su root to become root
(bug in Ultrix 4.0)
DanBoneh
Many ways to escape jail as root• Create device that lets you access raw disk
• Send signals to non chrooted process
• Reboot system
• Bind to privileged ports
DanBoneh
Freebsd jailStronger mechanism than simple chroot
To run: jail jail-path hostname IP-addr cmd
– calls hardened chroot (no “../../” escape)
– can only bind to sockets with specified IP address and authorized ports
– can only communicate with processes inside jail
– root is limited, e.g. cannot load kernel modules
DanBoneh
NotallprogramscanruninajailProgramsthatcanruninjail:• audioplayer• webserver
Programsthatcannot:• webbrowser• mailclient
DanBoneh
Problems with chroot and jailCoarse policies:– All or nothing access to parts of file system– Inappropriate for apps like a web browser
• Needs read access to files outside jail (e.g. for sending attachments in Gmail)
Does not prevent malicious apps from:– Accessing network and messing with other machines– Trying to crash host OS
DanBoneh
Isolation
SystemCallInterposition
DanBoneh
System call interpositionObservation: to damage host system (e.g. persistent changes) app must make system calls:
– To delete/overwrite files: unlink, open, write– To do network attacks: socket, bind, connect, send
Idea: monitor app’s system calls and block unauthorized calls
Implementation options:– Completely kernel space (e.g. GSWTK)– Completely user space (e.g. program shepherding)– Hybrid (e.g. Systrace)
DanBoneh
Initial implementation (Janus) [GWTB’96]
Linux ptrace: process tracingprocess calls: ptrace (… , pid_t pid , …)and wakes up when pid makes sys call.
Monitor kills application if request is disallowed
OSKernel
monitoredapplication(browser)
monitor
user space
open(“/etc/passwd”, “r”)
DanBoneh
Complications• If app forks, monitor must also fork– forked monitor monitors forked app
• If monitor crashes, app must be killed
• Monitor must maintain all OS state associated with app
– current-working-dir (CWD), UID, EUID, GID
– When app does “cd path” monitor must update its CWD• otherwise: relative path requests interpreted incorrectly
cd(“/tmp”)open(“passwd”,“r”)
cd(“/etc”)open(“passwd”,“r”)
DanBoneh
Problems with ptracePtrace is not well suited for this application:– Trace all system calls or none
inefficient: no need to trace “close” system call – Monitor cannot abort sys-call without killing app
Security problems: race conditions– Example: symlink: me ⟶ mydata.dat
proc 1: open(“me”)monitor checks and authorizesproc 2: me ⟶ /etc/passwdOS executes open(“me”)
Classic TOCTOU bug: time-of-check / time-of-use
time
not atomic
DanBoneh
Alternate design: systrace [P’02]
• systrace only forwards monitored sys-calls to monitor (efficiency)
• systrace resolves sym-links and replaces sys-call path arguments by full path to target
• When app calls execve, monitor loads new policy file
OSKernel
monitoredapplication(browser)
monitor
user space
open(“etc/passwd”, “r”)
sys-callgateway
systracepermit/deny
policy filefor app
DanBoneh
Ostia:adelegationarchitecture[GPR’04]Previousdesignsusefiltering:• Filterexaminessys-callsanddecideswhethertoblock• Difficultywithsyncingstatebetweenappandmonitor(CWD,UID,..)
– Incorrectsyncingresultsinsecurityvulnerabilities (e.g.disallowedfileopened)
Adelegationarchitecture:
OSKernel
monitoredapplication agent
user space
policy filefor app
libcopen(“etc/passwd”, “r”)
DanBoneh
Ostia:adelegationarchitecture[GPR’04]• Monitoredappdisallowedfrommakingmonitoredsyscalls
– Minimalkernelchange(…butappcancallclose()itself)
• Sys-calldelegatedtoanagentthatdecidesifcallisallowed– Canbedonewithoutchangingapp
(requiresanemulationlayerinmonitoredprocess)
• Incorrectstatesyncingwillnotresultinpolicyviolation
• Whatshouldagentdowhenappcallsexecve?– Processcanmakethecalldirectly.Agentloadsnewpolicyfile.
DanBoneh
PolicySample policy file:
path allow /tmp/*path deny /etc/passwdnetwork deny all
Manually specifying policy for an app can be difficult:– Systrace can auto-generate policy by learning how app
behaves on “good” inputs– If policy does not cover a specific sys-call, ask user
… but user has no way to decide
Difficulty with choosing policy for specific apps (e.g. browser) is the main reason this approach is not widely used
DanBoneh
NaCl: a modern day example
• game: untrusted x86 code
• Two sandboxes:– outer sandbox: restricts capabilities using system call interposition
– Inner sandbox: uses x86 memory segmentation to isolateapplication memory among apps
Browser
HTMLJavaScript
NaClruntime
game
DanBoneh
Isolation
IsolationviaVirtualMachines
DanBoneh
Virtual Machines
Virtual Machine Monitor (VMM)Guest OS 2
Apps
Guest OS 1
Apps
HardwareHost OS
VM2 VM1
Example: NSA NetTopsingle HW platform used for both classified and unclassified data
DanBoneh
Why so popular now?VMs in the 1960’s:– Few computers, lots of users– VMs allow many users to shares a single computer
VMs 1970’s – 2000: non-existent
VMs since 2000:– Too many computers, too few users
• Print server, Mail server, Web server, File server, Database , …– Wasteful to run each service on different hardware– More generally: VMs heavily used in cloud computing
DanBoneh
VMM security assumptionVMM Security assumption:– Malware can infect guest OS and guest apps– But malware cannot escape from the infected VM• Cannot infect host OS• Cannot infect other VMs on the same hardware
Requires that VMM protect itself and is not buggy– VMM is much simpler than full OS
… but device drivers run in Host OS
DanBoneh
Problem: covert channels• Covert channel: unintended communication channel
between isolated components– Can be used to leak classified data from secure
component to public component
ClassifiedVM PublicVM
secretdoc
malw
are
listenercovertchannel
VMM
DanBoneh
An example covert channelBoth VMs use the same underlying hardware
To send a bit b ∈ {0,1} malware does:– b= 1: at 1:00am do CPU intensive calculation– b= 0: at 1:00am do nothing
At 1:00am listener does CPU intensive calc. and measures completion time
b = 1 ⇒ completion-time > threshold
Many covert channels exist in running system:– File lock status, cache contents, interrupts, …– Difficult to eliminate all
DanBoneh
SupposethesysteminquestionhastwoCPUs:theclassifiedVMrunsonone andthepublicVMrunsontheother.
IsthereacovertchannelbetweentheVMs?
Therearecovertchannels,forexample,basedonthetimeneededtoreadfrommainmemory
DanBoneh
VMM Introspection: [GR’03]
protecting the anti-virus system
DanBoneh
Intrusion Detection / Anti-virusRuns as part of OS kernel and user space process
– Kernel root kit can shutdown protection system– Common practice for modern malware
Standard solution: run IDS system in the network– Problem: insufficient visibility into user’s machine
Better: run IDS as part of VMM (protected from malware)– VMM can monitor virtual hardware for anomalies– VMI: Virtual Machine Introspection
• Allows VMM to check Guest OS internals
DanBoneh
InfectedVMmalw
are
VMM
GuestOS
Hardware
IDS
DanBoneh
Sample checksStealth root-kit malware:
– Creates processes that are invisible to “ps”– Opens sockets that are invisible to “netstat”
1. Lie detector check– Goal: detect stealth malware that hides processes
and network activity– Method:
• VMM lists processes running in GuestOS• VMM requests GuestOS to list processes (e.g. ps)• If mismatch: kill VM
DanBoneh
Sample checks2. Application code integrity detector
– VMM computes hash of user app code running in VM– Compare to whitelist of hashes
• Kills VM if unknown program appears
3. Ensure GuestOS kernel integrity– example: detect changes to sys_call_table
4. Virus signature detector– Run virus signature detector on GuestOS memory
DanBoneh
Isolation
Subvirting VMIsolation
DanBoneh
Subvirt [King et al. 2006]
Virus idea:– Once on victim machine, install a malicious VMM– Virus hides in VMM– Invisible to virus detector running inside VM
HW OS
⇒
HW
OS VMM and virus
anti-virus
anti-virus
DanBoneh
The MATRIX
DanBoneh
DanBoneh
VM Based Malware (blue pill virus)• VMBR: a virus that installs a malicious VMM (hypervisor)
• Microsoft Security Bulletin: – Suggests disabling hardware virtualization features
by default for client-side systems
• But VMBRs are easy to defeat– A guest OS can detect that it is running on top of VMM
DanBoneh
VMM DetectionCan an OS detect it is running on top of a VMM?
Applications:
– Virus detector can detect VMBR
– Normal virus (non-VMBR) can detect VMM• refuse to run to avoid reverse engineering
– Software that binds to hardware (e.g. MS Windows) can refuse to run on top of VMM
– DRM systems may refuse to run on top of VMM
DanBoneh
VMM detection (red pill techniques)• VM platforms often emulate simple hardware
– VMWare emulates an ancient i440bx chipset… but report 8GB RAM, dual CPUs, etc.
• VMM introduces time latency variances– Memory cache behavior differs in presence of VMM– Results in relative time variations for any two operations
• VMM shares the TLB with GuestOS– GuestOS can detect reduced TLB size
• … and many more methods [GAWF’07]
DanBoneh
VMM DetectionBottom line: The perfect VMM does not exist
VMMs today (e.g. VMWare) focus on:Compatibility: ensure off the shelf software worksPerformance: minimize virtualization overhead
• VMMs do not provide transparency
– Anomalies reveal existence of VMM
DanBoneh
Isolation
SoftwareFaultIsolation
DanBoneh
Software Fault Isolation [Whabe et al., 1993]
Goal: confine apps running in same address space– Codec code should not interfere with media player– Device drivers should not corrupt kernel
Simple solution: runs apps in separate address spaces– Problem: slow if apps communicate frequently• requires context switch per message
DanBoneh
Software Fault IsolationSFI approach:
– Partition process memory into segments
• Locate unsafe instructions: jmp, load, store– At compile time, add guards before unsafe instructions– When loading code, ensure all guards are present
codesegment
datasegment
codesegment
datasegment
app #1 app #2
DanBoneh
Segment matching technique• Designed for MIPS processor. Many registers available.
• dr1, dr2: dedicated registers not used by binary– compiler pretends these registers don’t exist– dr2 contains segment ID
• Indirect load instruction R12 ⟵[R34] becomes:dr1 ⟵ R34scratch-reg ⟵ (dr1 >> 20) : get segment IDcompare scratch-reg and dr2 : validate seg. IDtrap if not equalR12 ⟵ [dr1] : do load
Guardensurescodedoesnot
loaddatafromanothersegment
DanBoneh
Address sandboxing technique• dr2: holds segment ID
• Indirect load instruction R12 ⟵[R34] becomes:
dr1 ⟵ R34 & segment-mask : zero out seg bitsdr1 ⟵ dr1 | dr2 : set valid seg IDR12 ⟵ [dr1] : do load
• Fewer instructions than segment matching… but does not catch offending instructions
• Similar guards places on all unsafe instructions
DanBoneh
Problem:whatifjmp [addr]jumpsdirectlyintoindirectload?
(bypassingguard)
Solution:
jmp guardmustensure[addr]doesnotbypassloadguard
DanBoneh
Cross domain callscaller
domaincallee
domain
call draw call stub draw:
return
braddrbraddrbraddr
ret stub
• Only stubs allowed to make cross-domain jumps• Jump table contains allowed exit points
– Addresses are hard coded, read-only segment
braddrbraddrbraddr
DanBoneh
SFI Summary• Shared memory: use virtual memory hardware
– map same physical page to two segments in addr space
• Performance– Usually good: mpeg_play, 4% slowdown
• Limitations of SFI: harder to implement on x86 :– variable length instructions: unclear where to put guards– few registers: can’t dedicate three to SFI– many instructions affect memory: more guards needed
DanBoneh
Isolation: summary• Many sandboxing techniques:
Physical air gap, Virtual air gap (VMMs),System call interposition, Software Fault isolationApplication specific (e.g. Javascript in browser)
• Often complete isolation is inappropriate– Apps need to communicate through regulated interfaces
• Hardest aspects of sandboxing:– Specifying policy: what can apps do and not do– Preventing covert channels
DanBoneh
THEEND
top related